LoadingPagedListAdapter.kt

open class LoadingPagedListAdapter<T, VH : RecyclerView.ViewHolder>(
    private val onOnCreateViewHolder: LoadingPagedListAdapter<T, VH>.(parent: ViewGroup, viewType: Int) -> VH,
    private val onOnBindViewHolder: LoadingPagedListAdapter<T, VH>.(holder: VH, position: Int) -> Unit,
    private val onGetItemViewType: LoadingPagedListAdapter<T, VH>.(Int) -> Int = { 0 },
    private val lifecycle: LiveData<DataSourceLifecycle>,
    differ: DiffUtil.ItemCallback<T>,
    private val onCreateLoadingViewHolder: (LoadingPagedListAdapter<T, VH>.(parent: ViewGroup, viewType: Int) -> RecyclerView.ViewHolder)? = null
) : PagedListAdapter<T, RecyclerView.ViewHolder>(differ) {
    companion object {
        const val LOADING_TYPE = 19841119
    }

    override fun onAttachedToRecyclerView(recyclerView: RecyclerView) {
        super.onAttachedToRecyclerView(recyclerView)
        lifecycle.observeForever {
            if (it == Loading) {
                notifyItemInserted(itemCount)
            } else {
                notifyItemRemoved(itemCount)
            }
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
        return if (viewType == LOADING_TYPE) {
            onCreateLoadingViewHolder?.invoke(parent, viewType) ?:
                    object : RecyclerView.ViewHolder(
                        LayoutInflater.from(parent.context)
                            .inflate( R.layout.loading_indicator, parent, false)
                    ) {}
        } else {
            onOnCreateViewHolder(parent, viewType)
        }
    }

    override fun getItemViewType(position: Int): Int {
        return if (isLoading() && position == itemCount - 1) {
            LOADING_TYPE
        } else {
            onGetItemViewType(position)
        }
    }

    @Suppress("UNCHECKED_CAST")
    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        (holder as? VH)?.let { onOnBindViewHolder(it, position) }
    }

    private fun isLoading(): Boolean {
        return lifecycle.value == Loading
    }
}